2 Answers
2

As already pointed out, the created date is shown on the summary anyway - but since your question is more about using hook_civicrm_summary(), here's an simple example to add the date above the existing summary content:

The contact created date does show up on Contact Summary page at the bottom of the summary page

If you want as a field to show in summary section then i feel you can use civicrm_summary hook but it would to be very difficult to manipulate where to insert a section in $content as it would be pure html. I would use Buildform and/or Page run hook to add field with some JS to place the field in the page/form.